1. Field of the Invention
The invention relates to the field of flexible wiring boards, particularly to a process for manufacturing a flexible wiring board capable of forming fine metal bumps and the flexible wiring board manufactured thereby.
2. Description of Related Art
Recently, there is an increasing demand for miniaturized semiconductor devices and a great importance is placed on flexible wiring boards on which a bare-chip semiconductor can be mounted.
FIGS. 4(a)-(d) is a processing diagram showing a process for manufacturing a flexible wiring board of the related art. Referring to the processing diagram, the process is explained in order. At first, a copper foil is applied on a polyimide film 111 and then the copper foil is patterned into a copper wiring 112 (FIG. 4(a)).
Then, the surface of polyimide film 111 is irradiated with laser beam 114 (FIG. 4(b)) to form openings 115 having a predetermined diameter (FIG. 4(c)). At this stage, the top surface of copper wiring 112 is exposed at the bottoms of openings 115, and then copper wiring 112 is plated with copper while the bottom surface is protected with a resin film 117 so that copper grows in openings 115 to form metal bumps 116.
When a bare-chip semiconductor device is to be mounted on such a flexible wiring board 110, an anisotropic conductive film is applied on metal bumps 116 and bonding pads of the semiconductor device are brought into contact with metal bumps 116 via the anisotropic conductive film and pressure is applied. Then, circuits within the semiconductor device contact copper wiring 112 via the anisotropic conductive film and metal bumps 116.
Flexible wiring boards of this type 110 are recently much used because they are thin and light and freely foldable to provide a high mounting flexibility.
However, residues of polyimide film 111 remain on the top surface of metal wiring 112 exposed at the bottoms of openings 115 when openings 115 are formed with laser beam 114 as described above. Residues are removed by immersing the assembly in a chemical solution after openings 115 have been formed. However, it becomes more difficult for the chemical solution to penetrate into openings 115 as openings 115 become finer, and therefore more difficult to remove residues.
If residues cannot be removed, copper deposition speed varies from opening 115 to opening 115, whereby homogeneous metal bumps 116 cannot be formed.
Another problem is variation in the diameter of fine openings 115 (about 40 xcexcm to 50 xcexcm) formed by irradiating a rigid polyimide film 111 with laser beam 114, resulting in variation in the diameter and height of metal bumps 116 which causes failure of connection with semiconductor chips.
Still another problem is that it is difficult to reduce the spot diameter of high power laser beam 114, which makes it impossible to form openings 115 having a diameter smaller than 40 xcexcm, contrary to the recent demand for finer openings 115.
An object of the invention is to provide a technique capable of forming fine metal bumps with good precision to overcome the above disadvantages of the related art.
To attain the above object, the invention provides a process comprising the steps of forming a mask film, patterned by exposure and development, on a metal foil and growing metal bumps on the metal foil exposed at the bottoms of openings in the mask film.
In the invention, the step of growing metal bumps is followed by the steps of removing the mask film, applying a liquid resin material to form a resin material coating on the surface of the metal foil on which the metal bumps have been formed, and then curing the resin material coating into a resin film.
In the invention, the resin material coating may consist of a plurality of layered coatings.
When the resin material coating consists of a plurality of layered coatings, at least the uppermost coating may be a thermoplastic coating.
In the invention, the surface of the resin material coating on the metal foil may be located below the height of the metal bumps.
In the invention, the height of said metal bumps from the surface of the resin film may be 35 xcexcm or less.
In the invention, the curing step may be preceded by the step of etching surface portions of the resin material coating.
In the invention, the resin material may be a liquid containing a polyimide precursor to form the resin film from a polyimide.
In the invention, the step of forming a resin film may be followed by the step of partially etching the metal foil from the bottom surface to form a patterned metal wiring.
In this case, a support film may be formed on the bottom surface of the metal wiring.
In the invention, the support film may be partially etched to expose desired regions of the metal wiring.
Said process may further comprise the steps of bringing bonding lands of a semiconductor chip into contact with the metal bumps and applying heat and pressure to allow the resin film to develop adhesiveness, whereby the semiconductor chip is bonded to flexible wiring board.
The invention also provides a flexible wiring board manufactured by the process as defined above.
Flexible wiring boards of the invention include those having a semiconductor device connected to the metal bumps.
As defined above, the invention relates to a process for manufacturing a flexible wiring board having metal bumps and the flexible wiring board manufactured thereby.
In the invention, an exposable dry film or resist film is applied or deposited on a metal foil and patterned by exposure and development to form a mask film.
The metal foil is exposed at the bottoms of openings in the mask film, so that metal bumps grow at exposed regions of the metal foil when the metal foil is immersed in a plating solution while its bottom surface is protected.
The openings in the mask film can be formed in a fine size with high precision by photolithography. Therefore, the metal bumps can also be homogeneously grown both in width and height.
Then, a liquid resin material is applied and dried or otherwise treated to form a resin material coating on the surface of the metal foil on which the metal bumps have been formed, after which the resin material coating is heated or otherwise cured into a resin film, whereby the surface of the metal foil on which the fine metal bumps have been formed can be covered with the resin film. If the resin material coating has a thickness smaller than the height of metal bumps, the tops of the metal bumps may project from the surface of the resin film without post-treatment.
If the resin material cover the tops of the metal bumps, the resin film is also formed by curing on the surfaces of the metal bumps, which can be, however, exposed by polishing or etching.
If etching is used, an uncured resin material coating can be etched to form a resin film with the tops of the metal bumps being exposed.
The resin film may be a thermosetting or thermoplastic film or a laminate of such films as far as it is flexible. From the viewpoint of durability or reliability, it is preferable that the resin material is a polyimide precursor to be cured into a polyimide film.
After the resin film has been formed, the bottom surface of the metal foil can be exposed and etched using a dry film or photoresist as a mask to give a copper wiring. Then, a support film can be formed on the bottom to protect the copper wiring, whereby a flexible wiring board having reliable insulating properties is obtained.
The resin film can be formed to have a multilayer structure by layering resin material coatings. If the uppermost layer of the resin film consists of a thermoplastic resin, the thermoplastic resin film develops adhesiveness upon heating to ensure bonding to a semiconductor device or the like without using anisotropic conductive film.
The support film may be formed by applying a sheet-like film or coating a resin material solution as defined above and curing it. The support film may be patterned to partially expose desired regions of the metal wiring for forming contact regions for connection with another flexible wiring board or contact regions for wire bonding.
Variation in the height of metal bumps grown by electroplating increases with size. Experiments show that the variation is limited to xc2x13 xcexcm when the height above the surface of the resin film is 35 xcexcm or less in contrast to xc2x15 to xc2x17 xcexcm observed when said height is 40 xcexcm. When a non-flexible material such as a semiconductor chip is to be connected to metal bumps, the yield is more influenced by variation than bump height.